The present invention relates to a device, process and apparatus for stamping a plastic material using a die to form a shaped article. In particular, it relates to process for stamping a detergent bar.
By xe2x80x9cdetergent barxe2x80x9d is meant a tablet, cake or bar in which the level of surface active agent, which comprises soap, synthetic detergent active or a mixture thereof, is at least 20 wt. % based on the bar.
In the manufacture of detergent bars, a preformed composition comprising all components of the bar is typically extruded from a nozzle to form a continuous xe2x80x9crodxe2x80x9d which is cut into smaller pieces of predetermined length, commonly referred to as xe2x80x9cbillets.xe2x80x9d These xe2x80x9cbilletsxe2x80x9d are then fed to a stamper or, alternatively, are given an imprint on one or more surfaces using, for example, a die of the same dimensions as the bar surface. The bar surface is hit with force by, eg., a mallet or a die in the shape of a roller,
Stampers typically have a die formed in two halves each with a surface which contacts the billet during the stamping operation. These surfaces are adapted to close to a preset separation distance, thereby compressing the billet between the die halves to give the bar its final shape and appearance, and then separate. Excess composition is squeezed out from the die halves as they close. This is commonly referred to as xe2x80x9cflash.xe2x80x9d The flash is then separated from the soap bar by transferring the bar through holes in a xe2x80x9cdeflashing plate.xe2x80x9d
Conventional die stamping machines include xe2x80x9cpin diexe2x80x9d shaped machines in which a pair of opposing die members or die halves meet during a compaction step and a xe2x80x9cbox diexe2x80x9d machine in which a pair of opposing die members stamp a bar held within a through-opening in a box frame but do not meet during compaction, the peripheral face of the bar being restrained by the box frame.
The die halves are often each provided with a die or ejector insert. These are normally held closed within the die halve by springs but can be pushed open by compressed air or mechanical means to assist in the release of the bar from the die. During closing of the die halves a vacuum can be applied to remove air trapped in the die cavity between the detergent bar and die surface and, in the case of rotary dies, this vacuum assists in retaining the bars in place during rotation.
Stamping of detergent bars using a die is carried out to give the bars a reproducible shape, smooth surface and/or to imprint a design such as a logo, trademark or the like onto at least part of a surface of the bar.
However, as a result of die-blocking, i.e. amounts of residual detergent left on die halves which builds up during continued use of the dies, bars are often formed with visible imperfections on their surfaces or they may not release from the die surface. Incomplete release of detergent bars is a particular problem for more xe2x80x9cstickyxe2x80x9d compositions made of milder surfactants.
Numerous solutions to these problems have been proposed. One solution involves chilling the die halves during the stamping operation. Others have involved use of elastomers, Watanabe, U.S. Pat. No. 5,332,190 discloses an elastic molding die composed of an elastomer laminate film, the innermost layer of which does not contain fillers.
In U.S. Pat. No. 5,269,997 it is proposed to provide each of two dies of a soap mold with an elastomeric septum stretched across their surfaces. Such a system would be complex to use at the speed required for commercial manufacture and a thin covering would be prone to tearing and logo reproduction would be expected to be poor.
WO 96/00278 discloses a device for stamping a detergent bar comprising a die, the die having at least one bar stamping surface wherein the bar stamping surface is provided with an elastomeric coating, the total thickness of the elastomeric coating being less than 200 microns. In a preferred embodiment, the elastomeric coating is the sole elastomeric material of the bar stamping surface.
Another solution is proposed in EP 276 971 and U.S. Pat. Nos. 4,793,959 and 4,822,273, which involve the use of two die members, each comprising a non-elastomeric and an elastomeric part. The elastomeric part, which contacts the soap bar during the stamping process, comprises an elastomer coating of at least 200 microns and having a modulus of elasticity within a specified range.
Adams et al., U.S. Pat. No. 4,822,273 is directed to a die for stamping detergent bars which has a facing layer of elastomer. Example 7 mentions surface chilling of metal dies.
Adams et al., U.S. Pat. No. 4,793,959 is directed to detergent bars stamped with a die having a facing layer of elastomer. Chilled dies are mentioned in example 7.
Adams et al., U.S. Pat. No. 5,236,654 is directed to the stamping of detergent bars using liquid cooled coating die members. Die cooling using a coolant at about xe2x88x9220xc2x0 C. is mentioned. It is stated that the systems previously selected by factory engineers achieved average die surface temperatures of approximately 15xc2x0 C. with a wide distribution of temperatures across the die surface.
Kataoka, U.S. Pat. No. 4,629,650 is directed to a process for producing a molded thermoplastic resin by interposing during molding a thermoplastic resin different from the thermoplastic resin to be molded as a skin layer between the die surface and the thermoplastic resin to be molded.
Uemura et al., U.S. Pat. No. 5,035,849 is directed to a process for producing a molded article. The surface of the mold is cooled with a releasing agent for powder molding.
Leslie, U.S. Pat. No. 5,269,997 is directed to a method and apparatus for stamping a blank of plastic material such as a soap blank. An elastomer septum may be placed across each die half between the blank and the die half so as to prevent plastic material from sticking to the die halves after stamping.
Mao, U.S. Pat. No. 3,761,047 is directed to a film having heat-insulating particles such as talc. In Example 1, 30% by volume talc having an average size of about 60 microns is dispersed in a polytetrafluorethylene resin solution.
Bates et al., U.S. Pat. No. 5,378,733 is directed to a sound attenuating polymeric composite which comprises polyurethane having a filler comprising powdered stainless steel.
Even where elastomeric coatings or chilled dies are employed, problems are encountered when the detergent used is a very mild surfactant and/or the detergent bar composition is soft and sticky.
It has been discovered that the problems attendant to stamping of detergent bars comprising very mild surfactants and/or particularly sticky and soft compositions can be alleviated by the use of an elastomer coated die wherein the elastomer includes a conductive filler such as a conductive steel powder. By loading the elastomer with conductive steel powder and adjusting the elastomer formulation, thermally conductive polymers can be achieved. As a result, the normally insulating effect of the coating is reduced and chilling of the dies can be facilitated. Application of an elastomer which contains thermally conductive additives to a soap bar gives the release capability of a conventional insulating elastomer with the surface quality of a chilled surface.
For a more complete understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of preferred embodiments and the accompanying drawings.